Work machines such as, for example, excavators, loaders, dozers, motor graders, and other types of heavy machinery use one or more hydraulic actuators to accomplish a variety of tasks. These actuators are fluidly connected to a pump on the work machine that provides pressurized fluid to chambers within the actuators. An electro-hydraulic valve arrangement is typically fluidly connected within a hydraulic circuit between the pump and the actuators to control a flow rate of pressurized fluid to and from the chambers of the actuators to facilitate movement of the actuators. Multiple separate makeup and pressure relief valves are generally associated with the electro-hydraulic valve arrangement to maintain required pressure levels within the hydraulic circuit. However, these multiple makeup and pressure relief valves add cost to the hydraulic circuit and reduce reliability of the hydraulic circuit.
One method of reducing cost and improving reliability of the hydraulic circuit is described in U.S. Pat. No. 5,868,059 (the '059 patent) issued to Smith on Feb. 9, 1999. The '059 patent describes a hydraulic circuit having four independent metering valves to control draining and filling functions of an associated hydraulic actuator. Each of the independent metering valves includes a main valve element movable to communicate a chamber of the actuator with either a drain port or a supply port. A control chamber is selectively communicated with an external source of pressurized pilot fluid or drained of the pressurized pilot fluid to initiate movement of the main valve element. Each of the independent metering valves associated with a draining function includes an integral relief valve means and an integral makeup valve means. The relief valve means functions to drain fluid from the hydraulic circuit when pressure within the circuit exceeds a predetermined value. The makeup valve means functions to direct pressurized fluid to the hydraulic circuit when pressure within the circuit drops below a predetermined value.
Although the hydraulic circuit described in the '059 patent may reduce cost and improve reliability by combining the relief and makeup functions into the independent metering valves, the hydraulic circuit may still be costly, lack efficiency, and may limit design flexibility. Specifically, because the independent metering valves of the '059 patent require an external source of pressurized pilot fluid, overall system cost may be significant. In addition, energy must be expended to operate the external source of pressurized pilot fluid, which may reduce efficiency of the hydraulic circuit. Further, because the hydraulic circuit of the '059 system requires a connection to the external pilot source, the location of the metering valves may be limited and the associated piping between the valves and the pilot source may further increase cost of the system.
The disclosed metering valve is directed to overcoming one or more of the problems set forth above.